Microporous membranes have been used for separation of materials for a number of applications. Such materials can be prepared from a variety of inorganic or organic materials. For instance, polyamide microporous membranes have been used for agglutination assays because of their hydrophilicity and separation properties.
U.S. Pat. No. 4,066,512 (issued Jan. 3, 1978 to Lai et al) describes membrane structures composed of microporous membranes having a coating of a water-insoluble protein such as zein or collagen and methods of making such structures. Enzymes are bound to the protein coating in order to provide a large, stable catalytic surface for biological reactions. Useful materials for preparing microporous membranes include cellulose esters and polyamides such as nylon.
While polyamide materials are useful as microporous membranes in catalytic membrane structures as described above, they present problems when used in immunoassays involving the reaction of one immunoreactive species with its receptor. Nylon membranes, in particular, are highly susceptible to nonspecific interactions with immunoreactive species, such as antibodies and antigens. That is, the immunoreactive species are likely to complex with sites on the membrane instead of with, or in the absence of, corresponding receptor molecules. Thus, when such species are immobilized on solid carrier materials (for example, polymeric beads) which are placed near the membranes in agglutination assays, unwanted interactions arise between the membranes and the carriers. These undesired interactions cause background interference and a reduction in assay sensitivity. It would be highly desirable to have membranes which do not interfere with the activity of immunoreactive species.
The water-insoluble proteins described in U.S. Pat. No. 4,066,512 might be useful for reducing the undesirable interactions with immunoreactive species. However, those proteins require the use of organic solvents in coating procedures and therefore give rise to additional problems and concerns associated with organic solvents, such as objectionable odors, flammability and waste disposal. It would therefore be desirable to reduce or eliminate the interactions described above without the problems attendant with organic coating solvents.